High performance fiber ropes and other types of lines are used in a variety of industrial applications such as mooring deep-water drilling and production platforms. Honeywell Spectra® fiber rope, a kind of synthetic fiber rope constructed from high performance polymers, is widely used in civil, military and off-shore industries. The synthetic fiber ropes have high strength-to-weight ratios and possess adequate stiffness.
In mooring applications, a loss in structural integrity of such mooring ropes affect the rope performance and can lead to rope failure resulting in partial or even total breakouts of platforms or vessels. The financial consequences of any mooring accident can be enormous. Strain accumulation and change along the rope during rope deployment is an essential key factor in rope failure. Detecting strain which may affect the performance of the rope should provide a reliable benchmark with which to estimate the remaining life and establish criteria for rope recertification or retirement.
Known techniques for detecting strain in fiber ropes employ fiber optic monitoring systems based on Brillouin scattering or optical time-domain reflectometry. In the former, one or more single-mode glass fibers are applied to the rope. Due to elongation limitation of glass fiber, a special wind pattern is applied to integrate the glass fiber into the measured rope and extend the elongation range of the fiber. The single-mode glass fiber solution has advantages in that it has relative long measurement range (5 km), good gauge length (1 m), relative good strain measurement resolution (0.3% over 1 m gauge) and good elongation measurement range (up to 9%). However, it suffers from easy breakage and requires using a complicated winding method and high source optical power into the fiber.
In optical time-domain reflectometry, one or more multi-mode plastic fibers are applied to the rope. Adopting a multi-mode plastic fiber optic method for measuring strain is advantageous in that the method has good reliability and no special winding pattern is required. However, this latter method suffers from short measurement range of rope (up to 1 km), long gauge length (10 m), poor strain measurement resolution (less than 1% over 1 m gauge), poor elongation measurement range (up to 7%) and requires the use of a special process to add the reflective interfaces between the gauge length of the rope. For example, a process of UV writing or cutting and splicing the optical fiber can be used to change the refraction index to add the reflective interfaces.
There is a continuing need to provide improved systems and methods for monitoring the performance of members, such as fiber ropes, in services which provide an early warning of the loss of structural integrity and, where appropriate, enhance safe deployment thereof.